Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: 55 Tony on April 28, 2018, 07:24:04 AM
-
I have a 17084226 on a 454 with a Comp "hot street machine" 11-450-8 cam and compression ratio is somewhere above 10:1, (I didn't know about CCing the heads till after it was together.) Cranking compression is 210psi in all cylinders. The cam actually calls for only 9.5:1. It runs pretty darn good. I can't find my notes on what the current primary jets and rods are, but the primary is pretty good according the the A/F gauge. At WOT it's about 13.5:1 - 14:1 and I'd like to get that closer to 12.5:1 Also, it has old iron 781 heads. I don't know how/if the higher compression effects target A/F ratio. If it matters, I have headers and an Edlebrock manifold.
The current secondary rods are DP's. A bit fat but I guess the short power tips are helping at WOT. So I'd like a suggestion on what rods to try to get richer. Laying around I have sets of:
CE, DH, CK, and CC's. The hanger is a J but who knows if it's been bent. I have a few other hangers, I know I bent one so I'd be willing to bend it to what ever is needed.
I won't be able to test this real soon because the rear is out to get new ends on it to get rid of the C-clips, and I'm changing from 3.42 to 4.10 gears. I'm real excited about trying those gears out! :) I bought some slicks and have been to a local track that was just re-opened after being closed for years. They don't even have a tree or anything to time you at the moment, but it's only $20 to get in and it's good for a rookie like me who has never drag raced before. When I took the rear apart I found that I had twisted an axle, they are only 30 spline but I've been told by quite a few people that I must have had bad axles. The other axle wore where the bearing rode on it.
Sorry for the lost post, just tried to give all the info you may need.
Tony
-
I'd try the CE metering rods, they are smaller than the DP's and will fatten it up a bit at full throttle. Also make sure that you are using a high flow N/S assembly and adequate fuel delivery to the carb or the results may not be as expected.......Cliff
-
Thanks Cliff, I'll do that. I have the large needle and seat so all is OK there. Otherwise the carb is pretty good, I can always go from idle to full throttle without the slightest bog and it's not too slow to open either.
When I rebuilt the motor, the guy helping me got me to advance the cam 2 degree's. I asked some if I should change it to straight up and they said no, by now the chain has stretched so it probably is straight up. I'd appreciate your thoughts on this. As I mentioned, the cranking compression is real close to 210psi in all cylinders. I don't suppose you can guesstimate by looks how many CC's these 781 heads are? It also appears that it's been decked also, but again I didn't know to measure that until AFTER it was together and in the car.
-
I've seen online anywhere between 119~122cc for 781 & 049 heads. I'm running a set of 781's and when I had them done I had the shop check them and they came out at 114. This is after I had them cut. They may also have been cut before I got them.
210 psi seems quite high. Does it rattle or ping? I'm assuming having the cam advanced is causing some of it. What cc dome on your pistons? You can look up the part number and find out.
I have a 20cc dome and a .024" head gasket and mine comes out to 9.8:1 compression using one of the compression calculators. I've got around 165 psi cranking compression. The piston is 20 in the hole. This is before I knew any better.
-
210 cranking pressure seems very high for that cam in a 454 with 10 to 1 compression.
Was the cam just advanced 2 degrees from straight up, or advanced after it was degrees 2 more degrees?
Comp specs out the cam at 290 degrees seat timing with 232 @ .050" on a 110LSA.
In a 454CID engine at 10 to 1 or a tad higher compression I'd expect around 170-180psi cranking pressure based on the engine builds I've done here with similar parts......Cliff
-
The cam was advanced 2 degrees total.
I run 92 or 93 octane, no knocks or pings although it doesn't like lower octane at all. Actually using a compression calculator guessing at the head cc's and guessing how much it's been decked, it said somewhere close to 10.25:1. But I *think* the two guesses where on the conservative side, that's why I showed the heads pictures. I thought someone who has seen a lot of 781's could guess mostly be how close to the valve the surface is. I'd love to see the same shot of a new 781 head. The second pic is really hard to see what I'm trying to show, but I'm trying to show how close the head surface is to the adjacent rough casting that isn't milled. If the head got shaved much more, that entire rough cast area would be milled too.
I found this in my notes.
Piston volume -25.70cc
head gasket .039 ... I think?
Was told it was milled considerably. From what I found on the net, they were 118cc new.
bored .030 over.
Decked, but I don't have any numbers on that.
I have checked the compression with another gauge that was the rubber push and hold type. It read about 180psi and then I couldn't hold it tight enough and the pressure would push the gauge out of the hole.
It's quite a difference from the old flat top pistons with a chamfered edge, then it had about 140 psi. And that is with the same gauge I'm using now.
I don't have a measurement for what ever it's called, the piston down in the cylinder, but I can say that with clay on the piston to check for valve clearance, it was getting sort of close to the minimum, whatever that is/was.
I don't see the numbers for the rockers I used, but I recall that they lift a bit higher than stock rockers, whatever that ratio is.
Pistons are speed pro L2465F30 https://www.summitracing.com/parts/slp-l2465f30
I don't know why, but even though I checked "notify me of replies", I don't get an email. I just happened to check in here and saw the new replies.
-
In a 454CID engine at 10 to 1 or a tad higher compression I'd expect around 170-180psi cranking pressure based on the engine builds I've done here with similar parts......Cliff
Cliff. Can you share some of your secrets on the 454 builds you have done? I will be replacing my 350 with a 454 with stock 781 heads in the future. Cam choice, pistons used, head mods, intake and of course, Q-jet needed. Street car.
Phil
-
For all engines built here we take steps to establish very tight quench in them. The target is .035" and never over .040" for any reason.
The parts used to build them would depend on the goals for the project and power level needed.
For a truck 454 we'd keep the compression ratio a bit lower and smaller cam on a wide LSA.
I typically don't use aftermarket intakes on most builds, the factory iron ones are fine, even the "flat" intakes that came on a lot of 454's. From idle to 5000rpm's it difficult to outrun one of those intakes with anything in the aftermarket, plus they keep things low for plenty of hood clearance in many applications that need it.
The factory Q-jets that came on the Big Block Chevy engines are excellent, all years aside the pre-1969 models. Those have the early hinge pin location and large float, so if the engine is going to be making big power a later carb would be a better choice.
Big Block Chevy engines in all configurations have tremendous potential to make power, and excellent parts selection for them.
For a "max-effort" pump gas street build we will usually start with the passenger car oval port heads, install 2.19/1.88" valves in them, a little clean up in the bowls, under the valves and short turns. I'll target the compression ratio around 10 to 10.5 to 1, then use a cam with pretty long seat timing, and 112 or 114LSA. Crower makes a cam (can't remember the part number at the moment) around 234/244 @.050" on a 112LSA that really works well in those builds. Decent idle, plenty of vacuum for power brakes, and strong power from idle to near 6000rpm's.
I avoid any of the "modern" lobe profiles for engines built here, or tight LSA. They require too much spring pressure to keep things happy above the lifters and aside from a "ratty" idle they just don't deliver in terms of power production.
Of course if you are into "bling" and want a really lumpy idle, low vacuum at idle, stinky exhaust and poor street manners there are PLENTY of cams out there that fit that bill........Cliff
-
Thanks for the advice on the BB build. Sometimes we do not realize how good some of the factory parts are. Phil
-
In a 454CID engine at 10 to 1 or a tad higher compression I'd expect around 170-180psi cranking pressure based on the engine builds I've done here with similar parts......Cliff
Phil
I'm not sure if you don't believe the compression I said I read or if you think the gauges are faulty ... or something else? I had hoped to loan a gauge from one of the local auto parts stores to see what it measures but I ran out of time and steam today. Will it make much difference if the engine is cold?
No one seems to be able to say from the pictures I posted, if the valve relief area is cut into from milling and shaving the head a lot over the years or if that is common from when they were new.
-
For those here, and to satisfy my own curiosity, I borrowed a gauge. At first I only got 185, then I remembered I didn't open the throttle! With that open it then read about 205. Quite frankly, I trust my gauge better. Oh, the engine was cold, I don't know how/if that affects the reading?
-
Pretty high pressure readings for 454 CID with a 230 @ .050" cam on a 110LSA. I'd suspect the true static compression is higher than you have calculated or the cam is advanced further than you think it is, or a little of both.......Cliff
-
Based on how the intake valve is poked out of the chamber. I would say those heads have been cut quite a bit. I found a pic of my 781's, but the files are to big to post. Mine dont poke out like in your pic. I also have the 2.19/1.85 valves and yours looks to be the original 2.09?
-
"Based on how the intake valve is poked out of the chamber. I would say those heads have been cut quite a bit."
That is exactly what I've been hoping to hear, one way or the other. Since it isn't knocking, is there any drawback of having too high of a compression ratio? I have the timing at a modest 16* (for a BBC anyway). I know a lot of BB are set to 18 or 19.
73ss, I'm going to send you my email if you could share that picture with me please.?
-
e-mail sent
-
Pretty high pressure readings for 454 CID with a 230 @ .050" cam on a 110LSA. I'd suspect the true static compression is higher than you have calculated or the cam is advanced further than you think it is, or a little of both.......Cliff
Yes, I believe the head was milled a little or lot more than I accounted for. I really wish I had known about cc'ing them. It was my first build ever. I have replaced timing chains before and I'm pretty darn sure it's 2* advanced and no more. In your opinion, if I have all the low end torque that I can use, would you suggest setting the cam straight up? It's not an easy job with a BBC in a 55 chevy. Although the cam says 1500 to 5500 and I redline it at 5500rpm anyway. Comp told me the 2* advance would lower the power band about 500rpm at each end, but I know the guys on the phone are hit and miss. Oh, and by the way, the motor has almost 8000 tough miles on it with no problems so I guess all in all I did a few things right?
-
Advancing camshafts to improve low end power is a highly inaccurate assumption.
It gets regurgitated over and over on the Boards, but I doubt if very many folks have actually done any testing in that area to see what really happens with moving cams around in terms of engine and vehicle performance.
It is true that advancing the cam or lowering ICL closes the intake sooner, but it also moves the exhaust to cylinder scavenging is effected, often adversely.
Several years ago I decided to test that sort of thing and moved the cam I was using in my 455 4 times to see what would really happen.
It started out installed at 109ICL per the cam card. I had many hundreds of runs on the car so accurate comparisons could be made. I also logged many thousands of street miles so would also compare street performance as well.
First move was to 111ICL. It really didn't effect it much anyplace, just a very slight reduction in vacuum at idle and maybe just a tiny bit "lazy" at very light throttle openings. At the track it ran almost exactly the same at every point as it did at 109ICL.
Next moved it to 113ICL, and that really took some "snot" out of it. Idle vacuum dropped a solid 2" at 750rpm's, noticeably "lazy" right off idle, and it lost nearly almost 2 tenths and 2 MPH at the track.
Moved it to 107ICL next. The vacuum at idle returned, and it "felt" little better on the street, with slightly improved throttle response right off idle. It didn't really "feel" any better past about 2000rpm's, but when you are working with a big 455 engine that makes TONS of torque anyhow, "seat of the pants" evaluations are a bit difficult right to start with. Anyhow, race day came and at the track it KILLED it everyplace, slower in 60' and ET and MPH at every point.
Lesson learned, advancing a cam doesn't necessarily improve low end power, engine power across the loaded rpm range, or vehicle performance at any level......FWIW......Cliff
-
So ... read the card, set it and forget it?
Something else I *think* I realized. Bare with me, I don't know cam shaft lingo. But some of the numbers on roller cam specs make it look a bit wild when comparing it to a flat tappet cam. But since the flat tappet is, well, flat, the cam lobe hits it sooner and holds it longer than with a roller on the same cam. Is that right?
-
There are limitations with flat cams not present with roller designs.
The roller cams can get the valves up to full lift quicker/sooner and hold them there longer.
Flat cams would chew up the lobes trying to do the same thing........Cliff
-
Let me ask a different way. When stating specs for cams, do they give the actual lift of the cam lobes or the lift of the lifter and rod as it travels over the cam lobes?
-
Cam specifications will usually have "advertised" specs and .050" numbers for comparison purposes.
They will also list lobe lift, and usually lift at the valve with a given rocker arm ratio.
The problem with "advertised" specs is that some companies will use a different open point for those numbers. This can make comparing camshafts a bit confusing as one company may "rate" their cams at .004" tappet lift and another at .006", for example.
The specs look confusing at a glance, but really all camshafts can be compared to each other if you know what specs to look at and what they mean.
For example I'll put up some cam specs below to describe what one may see on a cam card.
Advertised duration: 280/290
.050" duration: 230/240
Lobe lift: .300"
Lift at the valves with 1.5 rocker arms: .450.
Lobe Separation Angle: 112
Intake Centerline (recommended): 108
You will also see intake opening and closing points @ .050" for most camshafts as well.
Most cam companies will grind their camshafts on "tight" LSA's. 110 is very common and many will go even tighter. The Lobe Separation Angle moves the lobes closer together. With any given cam this increases overlap vs moving them further apart. It also typically places the intake advanced further and closes the intake valve sooner in the cycle.
Factory camshafts will typically have much wider LSA's. This reduces overlap and improves vacuum at idle, power right off idle and broad/smooth/flat power curve.
Tighter LSA does pretty much the opposite. It decreases vacuum production at low rpms/idle speed, even to a point where it causes "miss-fire" or "lope". Stinky exhaust is another by-product of tight LSA. Tighter LSA also narrows up the power curve, pulls VE down in the RPM range, and increases cylinder pressure at VE.
It is my opinion that companies grinding camshafts are playing into the "bling" by producing camshafts that put some "attitude" in the idle quality. I certainly can't argue with that, nothing sounds better than your "fresh" engine build "thumping" pretty hard at idle like it's ready to make a hard run down the track.
The problem is that folks associate improved performance with a "rough" idle, and to be perfectly honest nothing could be further from the truth.
Engine power is a product of the CID, compression ratio, head flow and camshaft being used. The intake and exhaust are also players here. So basically the camshaft is simply a small part of a big plan, and not the deciding factory by itself as to how successful the end result will be.
For most street driven engines we use and prefer cams with wider LSA and longer seat timing. I also prefer higher compression even though many folks have bought into the nonsense we read daily on the NET about 9.5 to 1 static compression as pretty much the limit for pump gas.
Higher compression can be your friend with these things as it allows for a larger cam to be used with less negatives at idle and low rpm's.
continued below......
-
I love leading by example with these things. Recently we were asked to help out a customer who just built and dyno'd his first Pontiac 455 engine build. Comp Cams recommended their XR276HR camshaft, basic specs are 224/230 @ .050" on a 110LSA. The big 455 had 250cfm ported heads on it and 9.3 to 1 compression. Despite the "low" compression ratio it pinged on the dyno with the tiny 224/230 cam in it and didn't make very impressive power.
I was asked to help out as EVERYONE involved with the project on their end was blaming the Q-jet. Instead of talking carburetors I had the engine builder install a much larger cam, 236/245 @.050" on a 114LSA. I had him set the ICL at 110 instead of 106 like the previous smaller cam. My cam also had much larger .381" lobes, so bigger everyplace than the tiny XR276HR cam.
They made the swap, more dyno runs and sent me the results. They also noted that the larger cam actually idled much "smoother" than the smaller one and not the first hint of detonation when they cranked some timing to it to make the best dyno numbers.......Cliff